The present invention generally relates to masks and exposure methods using the masks, and more particularly to a mask which is used to form a desired pattern by use of a charged particle beam such as an electron beam and an exposure method which uses the mask to efficiently and stably form the desired pattern.
Heretofore, photolithography mainly has been used for forming fine patterns. But recently, due to the further increase in the integration density of integrated circuits, new exposure techniques have been developed and reduced to practice. The new exposure techniques use an X-ray and a charged particle beam such as an electron beam.
The charged particle beam exposure forms a pattern by use of a charged particle beam such as an electron beam which can be controlled electro-magnetically. One notable feature of the charged particle beam exposure is the fact that it is possible to form fine patterns in the submicron order. The charged particle beam exposure can generally be classified into a Gaussian beam exposure and a shaped beam exposure. These exposure techniques can further be divided into a system of drawing while continuously moving a substrate by continuously moving a stage and a step-and-repeat system of drawing while moving the substrate a predetermined amount for every drawing region.
The trend to further increase the integration density of integrated circuits is rapidly accelerating. Compared to the optical beam, the diameter of the minimum spot which can be formed by the electron beam is far smaller than that achievable by the optical beam. For this reason, the electron beam exposure fully satisfies the demand for increased integration density from the point of view of the resolution. However, from the practical point of view, there is a problem in that the exposure time is long. This problem of long exposure time is caused by the fact that the charged particle beam exposure method draws in one stroke.
In the semiconductor device production apparatus which employs the electron beam exposure method, various methods have been proposed to reduce the exposure (drawing) time. One proposed method exposes the pattern in parts by use of a transmission mask which has predetermined patterns so as to reduce the exposure time. Another method uses a transmission mask which has a pattern formed for the entire exposure region and exposes the pattern in one shot. Still another method uses a transmission mask which has patterns for exposing a selected pattern using a shaped electron beam with a rectangular shape, a square shape, a triangular shape or the like.
For example, a Japanese Laid-Open Patent Application No. 52-119185 proposes a method of selectively irradiating an electron beam on a transmission mask which has different patterns formed at different block positions, and one block is exposed in one shot. A method of exposing a desired pattern by use of a transmission mask which is formed with the desired pattern in its entirety is reported in J. Vac. Sci. Technol. B3(1), Jan/Feb 1985(140). A method of forming the transmission mask which may be used in these methods is reported in J. Vac. Sci. Technol. B3(1), Jan/Feb 1985(58). A method of reducing the pattern to 1/10 similarly to the step-and-repeater of an ion beam exposure is reported in J. Vac. Sci. Technol. B3(1), Jan/Feb 1985(194). Furthermore, a Japanese Laid-Open Patent Application No. 62-260322 proposes an electron beam exposure apparatus which makes an exposure using a stop plate having rectangular holes for forming a repetition pattern which is necessary for forming a memory cell or the like and a general rectangular pattern.
Out of the above proposed methods, the method proposed in the Japanese Laid-Open Patent Application No. 62-260322 was most suited for reducing the exposure time.
FIG. 1 is a perspective view of a semiconductor device production apparatus for explaining a conventional electron beam exposure method proposed in the Japanese Laid-Open Patent Application No. 62-260322. In FIG. 1, the semiconductor device production apparatus comprises an electron gun 51, a first rectangular aperture 52, a convergent lens 53, and an electrostatic deflector 54 for selecting a mask pattern and varying a transmitting beam size. The deflector 54 has a function of moving an electron beam and at least one such deflector is normally necessary when making the exposure with a variable rectangular shaped beam. In FIG. 1 there are also shown a transmission mask 55 which is also referred to as a stencil mask, a design pattern 56, and a wafer 57 which is to be subjected to the exposure.
The design pattern 56 is preformed in the transmission mask 55. This design pattern 56 may be formed by a variable shaped beam which has a rectangular shape, a square shape, a triangular shape or the like, for example. Transmission holes through which the electron beam is transmitted are formed in the transmission mask 55, and groups of design patterns 56 are formed within the same layer as these transmission holes. A convergent lens (not shown) is provided between the deflector 54 and the transmission mask 55.
A description will now be given of the operating principle of this conventional semiconductor device production apparatus. As shown in FIG. 1, the transmission mask 55 which comprises the groups of design patterns 56 within the same layer as the electron beam shaping or exposure is located at a position where the electron beam passes, so that the exposure is made by selectively irradiating the electron beam on the groups of design patterns 56. The group of design patterns 56 on the transmission mask 55 is selected by deflecting the electron beam to a focal point position by use of the deflector 54. The pattern is exposed on the wafer 57 by the appropriate projection of the electron beam which is transmitted through the transmission holes in the transmission mask 55.
The method of providing the plurality of patterns on the transmission mask and deflecting the electron beam depending on the pattern to be used for the exposure is an effective method from the point of view of realizing a high-speed exposure. But when the electron beam is deflected by a deflecting system, an image distortion is inevitably introduced at the wafer surface. In other words, even when the electron beam is deflected to irradiate the same position on the wafer through two patterns of the transmission mask located at mutually different positions, the irradiated position on the wafer becomes different for the two cases, that is, different for the two patterns used. Therefore, there is a need to correct the irradiating position of the electron beam on the wafer depending on the pattern used, that is, the position of the pattern in the transmission mask.
It is possible to detect the irradiating position of the electron beam on the wafer by use of a reflected electron beam detector which detects the electron beam which is reflected at the wafer surface. However, because the patterns of the transmission mask are complex and mutually different, it is impossible to accurately detect whether or not the irradiating position of the electron beam through the two patterns actually coincide on the wafer. Hence, an operator monitors an image on a scanning electron microscope (SEM) to approximately determine the image distortion based on the images obtained for the two patterns of the transmission mask, but it takes time to make the required measurements and the measured results are not very accurate. Further, the irradiating position of the electron beam on the wafer through a pattern of the transmission mask inevitably changes with time even when the same pattern is used due to impurities and the like which adhere on the transmission mask. But the efficiency of the exposure apparatus becomes extremely poor if the above described measurements must be carried out frequently to correct the image distortion prior to the actual exposure.